The present invention relates to a brushless direct current motor having permanent magnets mounted to a rotor and commutated stator windings. More particularly, the present invention relates to a brushless direct current motor having adjustable speed and torque characteristics.
Brushless direct current motors with electronically commutated stator windings and permanent magnets mounted to the rotor are currently a subject of much interest. These motors provide a means of meeting the increasing demand for controllable, high-speed, low-maintenance motors for an ever increasing range of power ratings. The motor includes an inverter that applies commutated voltage and current to each phase of the stator windings. The speed of the motor is controlled by controlling the voltage amplitude applied to the stator windings, while the torque output from the motor is linearly proportional to the current through the windings. Commonly, the stator windings are connected either in a delta electrical configuration or a wye electrical configuration, depending on the desired motor performance. For example, if maximum speed is desired from a motor of a given horsepower rating, the stator windings are hard wired in a delta electrical configuration. If, on the other hand, maximum torque is desired from the motor, the stator windings are hard wired in a wye electrical configuration.
In many applications, however, it is not possible to choose a brushless direct current motor of the prior art that will have optimum motor characteristics for a given mechanical load. This is the case since many mechanical loads are not constant over time. Rather, both the operating speed and, more importantly, the torque required from the motor will vary over time. When motors are used in manufacturing processes, it is generally desirable to have high operating speeds in order to increase the rate of production of goods. Since large amounts of torque will also be required from the motor, the motor is generally over-rated for its intended use. In a brushless direct current motor, expensive inverters capable of handling the increased current necessary to generate the torque must be used. Likewise, larger diameter wires must also be used throughout the motor to handle the increased current, and additional heat dissipating devices must be used to dissipate the heat generated from these larger components. Accordingly, the size, weight and complexity of the motor will increase, increasing its cost and thereby making it less attractive for the intended use.